Negative blood test but still pregnant??

So last week I posted about how my period was 4 days late (which was last Tuesday), and I was receiving all negative home pregnancy tests so I went to an obgyn to have a blood test done which also came back negative. I've never had this problem with my period not showing up before so they told me to come back in two weeks which would be next week for a "problem" visit so they can check me for infections or other problems. So about 4 days ago I started having a pains in my stomach. Like a sharp pain that would come and go, so i thought maybe AF was finally on her way. When she didn't come I decided to take a pregnancy test for the heck of it (3 days ago which makes me 13 days late) and surprisingly I received a faint second line on my pregnancy test so I took about 4 more tests using different brands and all had the same faint second line!! Could I be pregnant? Was the blood test wrong? I thought they were supposed o be dead-on accurate?? Has anyone else experienced this or know someone who has??